Microporous polyolefin membranes are widely used for various applications such as battery separators used for lithium secondary batteries, nickel-hydrogen batteries, nickel-cadmium batteries, polymer batteries, etc., electrolytic capacitor separators, various filters such as reverse osmosis filtration membranes, ultrafiltration membranes, microfiltration membranes, etc., moisture-permeable, waterproof clothes, medical materials, etc. When the microporous polyolefin membrane is used for battery separators, particularly lithium ion battery separators, its performance largely affects the properties, productivity and safety of batteries. Accordingly, the microporous polyolefin membrane is required to have excellent mechanical properties, heat resistance, permeability, dimensional stability, shutdown properties, meltdown properties, etc.
In general, microporous membranes composed only of polyethylene have low meltdown temperatures, while microporous membranes composed only of polypropylene have high shutdown temperatures and poor mechanical strength. Accordingly, microporous membranes comprising polyethylene and polypropylene as main components are preferably for battery separators. To provide microporous membranes with improved mechanical strength, ultra-high-molecular-weight polyolefin is preferably added. For instance, JP3-64334A proposes a method for producing a microporous polyolefin membrane comprising (1) forming a gel-like molding comprising a composition containing ultra-high-molecular-weight polyolefin such that the ratio of (weight-average molecular weight/number-average molecular weight) is in a desired range, and a membrane-forming solvent, (2) stretching the gel-like molding without removing the solvent, and then (3) removing the solvent.
Japanese Patent 3348965 proposes a biaxially oriented microporous film made of a composition comprising 20-80% by mass of high-molecular-weight polyethylene having an intrinsic viscosity [η] of 10 dl/g or more, and 80-20% by mass of high-molecular-weight polypropylene having an intrinsic viscosity [η] of 3-15 dl/g, and having excellent closing temperature properties, rupture temperature properties, rigidity, tensile strength and air permeability.
However, microporous membranes comprising polyethylene and polypropylene, particularly those containing ultra-high-molecular-weight polyolefin, tend to be poor in such properties as thickness uniformity, etc. For instance, when ultra-high-molecular-weight polyethylene is added to microporous polyolefin membranes comprising high-molecular-weight polyethylene and high-molecular-weight polypropylene, the membranes tend to have sharkskin surfaces (see JP6-96753A). When microporous membranes having poor thickness uniformity are used as battery separators, they are likely to suffer short-circuiting, poor production yield, and poor cyclability due to low compression resistance.
In such circumstances, a microporous polyolefin membrane comprising polypropylene having MFR of 2.0 or less, and a polyethylene composition with weight-average molecular weight/number-average molecular weight of 8-100, the polypropylene content being 20% or less by mass was proposed (JP2002-194132A). Specifically, JP2002-194132A discloses a microporous polyolefin membrane comprising 30% by mass of powdery ultra-high-molecular-weight polyethylene having a weight-average molecular weight of 2.0×106, 65% by mass of powdery high-density polyethylene having a weight-average molecular weight of 3.0×105 (a polyethylene composition composed of the ultra-high-molecular-weight polyethylene and the high-density polyethylene having Mw/Mn of 20.5), and 5% by mass of a pelletized propylene homopolymer having a weight-average molecular weight of 6.0×105 and MFR of 0.5. This microporous polyolefin membrane has excellent thickness uniformity and good production yield.
As microporous polyolefin membranes having well-balanced thickness uniformity, heat resistance, mechanical properties, permeability, dimensional stability, shutdown properties, meltdown properties, and compression resistance, JP2004-196870A proposes microporous polyolefin membrane composed of polyethylene, and polypropylene having a weight-average molecular weight of 5×105 or more and a heat of fusion (measured by differential scanning calorimeter) of 90 J/g or more, the polypropylene content being 20% or less by mass, and JP2004-196871A proposes a microporous polyolefin membrane composed of polyethylene, and polypropylene having a weight-average molecular weight of 5×105 or more, and a melting point (measured by a differential scanning calorimeter at a temperature-elevating speed of 3-20° C./min) of 163° C. or higher, the polypropylene content being 20% or less by mass. However, the meltdown properties of any of the microporous polyolefin membranes disclosed by JP2002-194132A, JP2004-196870A and JP2004-196871A are not necessarily satisfactory, because of the polypropylene content of 20% or less by mass.
If the polypropylene content were increased in microporous polyolefin membranes comprising polyethylene and polypropylene, until sufficient meltdown properties (specifically meltdown temperatures higher than 175° C.) can be obtained, the membranes would be likely to have decreased permeability and prick resistance. Thus, the microporous membranes for battery separators comprising polyethylene and polypropylene are required to have high meltdown temperatures and well-balanced permeability and mechanical properties.